Lecture 21: Structure of ordinary-crystalline deformation ring for l = p

Transcription

1 Lecture 21: Structure of ordinary-crystalline deformation ring for l = p 1. Basic problem Let Λ be a complete discrete valuation ring with fraction field E of characteristic 0, maximal ideal m = (π), and residue field k of characteristic p > 0; we will ultimately be interested in the case when k is finite (and in particular, perfect). Consider a complete local noetherian Λ-algebra R with residue field k, so R = Λ[x 1,..., x m ]/(f 1,..., f s ), and suppose there is given a continuous representation ρ : G K GL n (R) for a p-adic field K (i.e., K is a finite extension of Q p ). Note that R[1/p] = R[1/π] = R Λ E; we call this the generic fiber of R over Λ, but beware that as an E-algebra this is typically very far from being finitely generated. For shorthand, we write R E to denote this generic fiber. We are going to be interested in certain subsets of MaxSpec(R E ). Recall from the lecture in the fall on generic fibers of deformation rings that the maximal ideals of R E are precisely the kernels of E-algebra homomorphisms R E E into finite extensions E /E, or equivalently Λ-algebra homomorphisms R E, and that such maps are necessarily given by h(x 1,..., x m ) h(a 1,..., a m ) for a i in the maximal ideal of the valuation ring Λ of E. In more geometric terms, MaxSpec(R E ) is identified with the zero locus {(a 1,..., a m ) E m a i < 1, f j (a 1,..., a m ) = 0 for all j} taken up to the natural action by Gal(E/E) on this locus. Remark 1.1. Loosely speaking, we view R E as an algebraist s substitute for working directly with the rigid-analytic space {f 1 = = f s = 0} inside of the open unit polydisk over E. There is a way to make this link more precise, by relating R E to the algebra of bounded analytic functions on this analytic space, but we do not need such a result so we will pass over it in silence; nonetheless, trying to visualize MaxSpec(R E ) in terms of this analytic zero locus is a good source of intuition. In the fall lecture on generic fibers of deformation rings, we recorded a few basic algebraic properties of R E and we recall them now. First, R E is noetherian and Jacobson; the latter means that every prime ideal is the intersection of the maximals over it, or equivalently the radical of any ideal is the intersection of the maximals over it. This ensures that focusing on MaxSpec does not lose a lot of information, much like algebras of finite type over a field (the classic example of a Jacobson ring). In contrast, a local ring of positive dimension (e.g., a discrete valuation ring, or R as above when not artinian!) is never Jacobson! An additional important property, already implicit in the preceding discussion, is that if x MaxSpec(R E ) 1

2 2 then the corresponding residue field E(x) = R E /m x is finite over E. It then makes sense to consider the specialization of ρ at x: ρ x : G K ρ E GL n (R E ) GL n (E(x)). Especially when E is finite over Q p, we visualize ρ as a family of p-adic representations {ρ x } with varying coefficient fields E(x) of finite degree over E. Remark 1.2. Note that each such ρ x is continuous (and so is a p-adic representation of G K ) since x carries R into the valuation ring of E(x) via a local map and ρ is continuous when R is given its local (i.e., max-adic) topology. For a property P of (isomorphism classes of) G K -representations over finite extensions of E and for any x MaxSpec(R E ), let P(x) denote the condition that ρ x satisfies property P. (In practice, P is always insensitive to finite scalar extension on the coefficient field over E.) It is useful to consider whether or not the locus P(R E ) = {x MaxSpec(R E ) P(x) holds} is analytic in the sense that it is cut out by an ideal J of R E. That is, for a Λ-algebra map x : R E to a finite extension E /E, does ρ x satisfy P if and only if x(j) = 0? A given ideal J in R E satisfies this condition if and only if its radical does (since E is reduced), so we may as well restrict attention to radical J. But since R E is Jacobson, a radical ideal J in R E is the intersection of the maximals over it, so in other words there is exactly one possibility for a radical J: J P := m x P(x) holds where m x = ker(x : R E E(x)). Note that if P(x) fails for all x MaxSpec(R E ) then J P = (1) (either by logic, convention, or the utiliarian reason that it is consistent with what follows). Turning this reasoning around, we take the above expression for J P as a definition, so V (J P ) := Spec(R E /J P ) is the Zariski closure of the locus of x MaxSpec(R E ) such that P(x) holds. The analyticity question for P then amounts to the following question: does every closed point of V (J P ) satisfy P? It is by no means clear how one could answer this question, and in the early days of modularity lifting theorems this was a serious problem which had to be treated by ad hoc methods depending on the specific P. One of the big achievements of Kisin s introduction of integral p-adic Hodge theory into Galois deformation theory is to provide systematic techniques for proving an affirmative answer to this question for many interesting P involving conditions related to p-adic Hodge theory (e.g., crystalline with Hodge-Tate weights in the interval [ 2, 5]). In any situation for which the P-analyticity question has an affirmative answer, to exploit it one needs to answer a deeper question: how can we analyze properties of R E /J P, such as regularity, dimension, connectedness of spectrum, etc.? Kisin s methods also gave a way to address this question. We will develop this for a special P that can be studied without the full force of p-adic Hodge theory: the ordinary crystalline deformation problem, to be defined later. A key insight that underlies Kisin s strategy for answering both of these questions is to use P to define a new moduli problem on arbitrary R-algebras (forgetting the topological

3 structure of R) which is shown to be represented by a proper (even projective) R-scheme Θ : X P Spec R such that: (1) the map Θ E : X P,E Spec(R E ) obtained by inverting p (equivalently, localizing by Λ E) is a closed immersion whose image has as its closed points precisely the ones which satisfy P (so this closed subscheme, after killing nilpotents, recovers J P and provides an affirmative answer to the P-analyticity question), (2) the Λ-scheme X P is formally smooth in a sense we will make precise later. (In practice X P is very far from being finite type over Λ, just like R itself, so we cannot naively carry over the notion of smooth morphism from algebraic geometry in terms of a Jacobian criterion.) An important consequence of condition (2) is that the generic fiber X P,E is formally smooth over E, which is to say that it is regular and hence reduced. (In geometric language, this says that the rigid-analytic space over E arising from R E /J P in the open unit polydisc is smooth.) In particular, X P,E = V (R E /J P ) (affine!), so the E-algebra R E /J P that we wish to understand is the coordinate ring of the affine generic fiber X P,E of the (typically nonaffine!) moduli scheme X P over R which we can try to study by moduli-theoretic reasoning. In fact, we will study the structure of the generic fiber over E by using moduli-theoretic considerations with the schemes X P mod πr and X P mod m R which live in characteristic p! Letting I P = ker(r R E /J P ) be the ideal of the Zariski closure in Spec R of the P- locus in Spec R E, the quotient R/I P is reduced with generic R E /J P. In practice we will think of R/I P as an integral parameter space for the property P. In particular, the formal smoothness over Λ in (2) justifies viewing X P as a resolution of singularities of Spec(R/I P ) (for which it has the same E-fiber). 2. Some commutative algebra and algebraic geometry Before we launch into the definition and study of Kisin s moduli problems on R-algebras and their applications to the study of the P-locus in MaxSpec(R E ), we digress to explain some general considerations in commutative algebra and algebraic geometry which will be used throughout his method. It will be clearer to carry out these general considerations now so that we will be ready for their applications later. We consider the following general setup. Let (Λ, E, π, k, R) be as above, and let f : X Spec R be a proper R-scheme. There are two reductions of f that will be of interest: the reductions f : X := X mod π Spec(R/πR), f 0 : X 0 := X mod m R Spec k modulo πr and modulo m R respectively. In particular, X 0 is a proper (hence finite type) scheme over the residue field k. The quotient R/πR is naturally a k-algebra (since k = Λ/(π)), but X is typically huge (not finite type) when thereby viewed as a k-scheme. Since f is proper, it carries closed points of X into closed points of Spec R. But there is only one closed point of the local Spec R, and X 0 is closed in X, so we conclude that the closed points of X coincide with those of X 0. Moreover, if x 0 X is a closed point then since it is closed in the scheme X 0 of finite type over k we see that the residue field κ(x 0 ) of X at x 0 (or equivalently, of X 0 at x 0 ) is of finite degree over k, 3

4 4 Now assume that k is perfect (e.g., finite). Consider a closed point x 0 X, so κ(x 0 )/k is a finite separable extension. Let Λ(x 0 ) be the unique (up to unique isomorphism) finite unramified extension of Λ with residue field κ(x 0 )/k. The completed local ring OX,x 0 is a Λ-algebra with residue field κ(x 0 )/k, so by Hensel s Lemma it admits a unique structure of Λ(x 0 )-algebra over its Λ-algebra structure. (This application of Hensel s Lemma crucially uses that we are working with the completed local ring and not the usual algebraic local ring O X,x0 ; this latter Λ-algebra is typically not a Λ(x 0 )-algebra in a compatible manner.) Hypothesis ( ): assume that O X,x 0 Λ(x 0 )[T 1,..., T n ] as Λ(x 0 )-algebras, for all closed points x 0 X 0. This hypothesis can be checked by means of functorial criteria, and that is how it will be verified in later examples of interest. It follows from ( ) that the completion O X,x 0 is regular, Λ-flat, and reduced modulo π for all x 0. These are the properties we will use to prove: Proposition 2.1. Under hypothesis ( ), the base change X over R/(π) is reduced and the total space X is regular and Λ-flat. Proof. We first handle the Λ-flatness, and then turn to the claims concerning reducedness modulo π and regularity. The π-power torsion in O X is a coherent ideal whose formation commutes with passage to stalks and completions thereof (by flatness of completion). For all closed points x 0, the completion of O X,x0 is Λ-flat by inspection of its assumed structure. Hence, the π-power torsion ideal has vanishing stalks at all closed points, so it vanishes on an open subset of X which contains all closed points. Such an open subset must be the entire space, so X is Λ-flat. By hypothesis ( ), each quotient O X,x 0 = O X,x 0 mod π is reduced, so the proper scheme X over the complete local noetherian ring R = R/(π) has reduced local rings at the closed points. Thus, the coherent radical of the structure sheaf of O X has vanishing stalks at all closed points, so exactly as for the Λ-flatness above we conclude that X is reduced. If the non-regular locus on X is closed then since the local rings on X at all closed points are regular (by inspection of their completions) it would follow that the non-regular locus is empty. That is, X is regular if the non-regular locus is closed. It remains to prove that the non-regular locus in X is Zariski-closed. The closedness of this locus in general locally noetherian schemes (and likewise for other properties defined by homological conditions) is a deep problem which was first systematically investigated by Grothendieck. His big discovery was that for a class of schemes called excellent the closedness always holds. He also proved that most noetherian rings which arise in practice are excellent. We refer the reader to [4, Ch. 13] for an elegant development of the basic properties of excellence (including the definition!), and here we just record the main relevant points: excellence is a Zariski-local property, it is inherited through locally finite type maps, and every complete local noetherian ring (e.g., every field, as well as R above) is excellent. Hence, the scheme X is excellent, so its non-regular locus is Zariski closed. Now we come to a very useful result which can be applied under the conclusions of the preceding proposition.

5 Lemma 2.2 (Reduced fiber trick). Let X be a Λ-flat R-scheme which is proper and for which X = X mod π is reduced. If X is connected and non-empty then X 0 = X mod m R and the generic fiber X E = X Λ E are both connected and non-empty. In general (without connectedness hypotheses), there is a natural bijective correspondence between connected components C 0 of X 0 and C E of X E by the requirement that C E is the E-fiber of the unique connected component of X with mod-m R fiber C 0. Proof. Since X is non-empty and Λ-flat, X E is non-empty. The theorem on formal functions, applied to the proper X over the complete local noetherian ring R, identifies the idempotents on X with those on X 0. In particular, X 0 is non-empty, and each connected component of X 0 uniquely lifts to a connected component of X. Hence, by passing to the connected components of X it suffices to prove that if X is connected then so is X E. The Λ-flatness of X implies that the ring O(X) of global functions on X injects into its localization O(X)[1/π] = O(X) E = O(X E ) which is the ring of global functions on X E. We assume that the latter contains an idempotent e and seek to prove e = 0 or e = 1. We can write e = e /π n for a minimal n 0 and a global function e on X. If n = 0 then e is idempotent on X and hence e = e {0, 1} since X is connected. Thus, we assume n 1 and seek a contradiction to the minimality of n. Since e 2 = e on X E, we can clear denominators (via Λ-flatness) to get e 2 = π n e on X. Thus, for e = e mod π we have e 2 = 0 on X. But X is reduced, so e = 0 on X. This says that e is divisible by π locally on X. Since X is Λ-flat, the local π-multiplier to get e is unique and hence globalizes. That is, e = πe for some e O(X). It follows that on X E we have contrary to the minimality of n. e = e π n = e π n 1, Inspired by the two preceding results, we are led to wonder: how can we ever verify Hypothesis ( )? We now present a functorial criterion. Proposition 2.3. Hypothesis ( ) holds if and only if for every artin local finite Λ-algebra B, X(B) X(B/J) is surjective. Proof. Fix a closed point x 0 X and let k /k be a finite Galois extension which splits k 0 := κ(x 0 ). Let Λ 0 = Λ(x 0 ), and let Λ be the finite unramified extension of Λ corresponding to k /k. Thus, (2.1) Λ Λ Λ 0 Λ j j:k 0 k where Λ j denotes Λ viewed as a Λ(x 0 )-algebra via the unique Λ-embedding Λ 0 Λ lifting the k-embedding j : k 0 k. Recall from above that OX,x 0 is canonically a Λ 0 -algebra. Let B be an artin local finite Λ 0 -algebra. A complete local noetherian Λ 0 -algebra with residue field k 0 is a formal power series ring over Λ 0 if and only if it is Λ 0 -flat with residue field k 0 and is regular modulo π. These properties hold if and only if the finite étale scalar extension by Λ 0 Λ yields the analogous properties using the residue field k, so it is equivalent to prove that this scalar 5

6 6 extension is a formal power series ring over Λ. In particular, the functorial criterion for the latter condition is precisely that the natural map of sets (2.2) Hom Λ0 (O X,x 0, B) Hom Λ0 (O X,x 0, B/J) is surjective for any artin local finite Λ -algebra B with residue field of finite degree over k 0 and any square-zero ideal J in B. We will reformulate this surjectivity in terms which are more easily related to the functor of points of X as we vary (B, J) with B an artin local finite Λ -algebra. Since B is artin local and Λ-finite, the natural restriction map Hom Λ (O X,x 0, B) Hom Λ (O X,x0, B) = X x0 (B) is bijective, where X x0 (B) denotes the set of Λ-maps Spec B X whose image is x 0. Using the Λ -algebra structure on B and the canonical Λ -algebra structure on O X,x 0, we also have the alternative description Hom Λ (O X,x 0, B) = Hom Λ (Λ Λ O X,x 0, B) = Hom Λ ((Λ Λ Λ 0 ) Λ0 O X,x 0, B). Using (2.1), this is identified with the disjoint union Hom Λ (Λ j,λ0 OX,x 0, B) = Hom Λ0 (OX,x 0, B j ) j:k 0 k j:k 0 k where B j denotes B viewed as a Λ -algebra via any g Gal(k /k) lifting j on k 0. The preceding identifications of Hom-sets are all functorial in B. In the final disjoint union above, as we vary through all pairs (B, J) with B an artin local finite Λ -algebra and J a square-zero ideal in B, the simultaneous surjectivity of (2.2) for all pairs (B j, J j ) s is thereby identified with the surjectivity of the natural map X x0 (B) X x0 (B/J) as B varies through artin local finite Λ -algebras. Recall that k /k is an arbitrary but fixed finite Galois extension which splits k 0 = κ(x 0 ). Thus, if X(B) X(B/J) is surjective for all artin local finite Λ-algebras B and square-zero ideals J B then Hypothesis ( ) holds. Conversely, if ( ) holds then for any such (B, J) we claim that X(B) X(B/J) is surjective. Pick a point in X(B/J). As a Λ-map Spec(B/J) X we claim that it hits a closed point x 0. Since B/J has residue field of finite degree over k, it suffices to show that this map lands in X 0 (as X 0 is a finite type k-scheme). Since the composite map Spec(B/J) Spec(R) over Λ lifts a point of R valued in a finite extension of k, it suffices to check that the only such point is the evident one which kills m R. Expressing R/(π) as the quotient of a power series ring over Λ, it suffices to prove that the only local k-algebra map from k [x 1,..., x m ] into a finite extension of k is evaluation at the origin. This is verified by restriction to the k-subalgebras k [x i ] for i = 1,..., m. The chosen point in X(B/J) identifies the residue field κ of B/J as a finite extension of the residue field k 0 at x 0, so B/J and hence B is thereby equipped with a natural structure of Λ 0 -algebra over its Λ-algebra structure. The chosen point in X(B/J) is thereby identified with a Λ-algebra map Spec(B/J) X hitting x 0 and lifting the specified extension structure

7 κ/k 0. This map corresponds to a local Λ-algebra map O X,x0 B/J lifting k 0 κ, which in turn uniquely factors through such a local Λ-map O X,x 0 B/J. This latter map is a Λ 0 -algebra map (as may be checked on residue fields). By ( ), the completion OX,x 0 is a formal power series ring over Λ 0, so its map to B/J lifts to a Λ 0 - algebra map OX,x 0 B. Running the procedure in reverse, this gives a Λ-map Spec B X which lifts the chosen point in X(B/J). Remark 2.4. The criterion in Proposition 2.3 is what was used in the verification of power series properties in the preceding lecture on the case l p. In Kisin s papers, he expresses things in terms of a much more general theory of formal smoothness for maps of topological rings, and he thereby invokes some very deep results of Grothendieck in this theory. For example, a noetherian algebra over a field of characteristic 0 is formally smooth (for the discrete topology) over that field if and only if it is regular. We will speak in the language of regularity and avoid any need for the theory of formal smoothness because we will appeal to general results in the theory of excellence (as was done in the proof of Proposition 2.1). The reader who is interested in reading up on the general theory of formal smoothness (such as its flatness aspects) should look at [5, Ch. 28], 17.5 in EGA IV 4, and (esp and ) in Chapter 0 IV of EGA. Certainly if one goes deeper into Kisin s techniques (beyond the ordinary crystalline deformation condition to be considered below) then it becomes important to use formal smoothness techniques in the generality considered by Kisin. The final topic we take up in this section is the algebro-geometric problem of giving a convenient criterion to prove that the proper map f E : X E Spec(R E ) is a closed immersion. More specifically, we want to give a criterion involving points valued in finite E-algebras C. Keep in mind that even though R E is a gigantic E-algebra in general, it is Jacobson and its maximal ideals have residue field of finite E-degree. In particular, the artinian quotients of R E at its maximal ideals are examples of such E-algebras C. The same goes for the R E -proper X E at its closed points (which lie over MaxSpec(R E ), due to the properness of f E ). Proposition 2.5. If f E is injective on C-valued points for all E-finite C then f E is a closed immersion. Proof. We will first prove that f E is a finite map (i.e., X E is the spectrum of a finite R E - algebra), so then we can use Nakayama s Lemma to check the closed immersion property. Since f E is proper, it suffices to prove that it is quasi-finite. For any map of finite type between noetherian schemes, the locus of points on the source which are isolated in their fibers (i.e., the quasi-finite locus ) is an open set: this is a special case of semi-continuity of fiber dimension. Thus, if f E has finite fibers over MaxSpec(R E ) then the open quasifinite locus of f E contains all closed points of X E (as these are precisely the points over MaxSpec(R E ), due to properness of f E ). But X E is a Jacobson scheme since it is finite type 7

8 8 over the Jacobson ring R E, and (as for any noetherian topological space) the only open set in X E which contains all closed points is the entire space. Hence, if f E has finite fibers over MaxSpec(R E ) then f E is a quasi-finite and therefore finite map. Letting C vary through the finite extension fields E /E, the injectivity of f E on E -valued points implies that the fiber of f E over each y MaxSpec(R E ) has only finitely many closed points. (Here we use that f 1 E (y) is closed in X E and is of finite type over E(y), with E(y) finite over E.) But a scheme of finite type over a field has finitely many closed points if and only if it is finite. Thus, f E indeed has finite fibers over MaxSpec(R E ). This argument even shows that such fibers have at most one physical point (since if a fiber contains two distinct points x x then using E containing E(x) and E(x ) makes f E fail to be injective on E -valued points). Now consider the finite map f E : X E Spec R E. To prove that the corresponding module-finite map of coordinate rings is surjective (so f E is a closed immersion), it suffices to check surjectivity after localizing at maximal ideals of R E. By Nakayama s Lemma, it is equivalent to check that the scheme-theoretic fiber Spec C Spec E(y) of f E over each y MaxSpec(R E ) satisfies C = 0 or C = E(y). The two composite maps Spec(C E(y) C) Spec C Spec E(y) coincide, so for the E-finite algebra C = C E(y) C we see that the composites Spec(C ) Spec C = X E Spec(RE ) Spec(E(y)) X E have the same composition with f E : X E Spec(R E ). By hypothesis, f E is injective on C -valued points! Hence, the projections Spec(C ) Spec(C) coincide, which is to say that the two inclusions C C = C E(y) C coincide. This easily forces C = E(y) if C 0 (by consideration of an E(y)-basis of C containing 1). 3. The ordinary crystalline deformation problem Now assume that k is finite! Let E /E be a finite extension and Λ its valuation ring. Fix a continuous representation ρ : G K GL 2 (E ) with cyclotomic determinant χ. We already know what it means to say that ρ is ordinary: this means that there is a G K -equivariant quotient line with action by an unramified character η. Such a quotient line is unique, as the Λ -valued det ρ = χ is ramified, so it is equivalent to say that ρ admits an I K -equivariant quotient line with trivial I K -action. This notion of ordinarity can be expressed in terms of a GL 2 (Λ )-valued conjugate of ρ by using saturated Λ -lines. In terms of a G K -stable Λ -lattice, we get an upper-triangular form for ρ, or equivalently for ρ IK, and this extension structure identifies ρ IK with a class in H 1 (I K, Λ (1)) = lim H 1 (I K, (Λ /(p n ))(1)) with Λ /(p n ) a finite free Z/(p n )-module since [k : F p ] is now assumed to be finite. This rank is equal to the Z p -rank of the finite free Z p -module Λ. By computing with a Z p -basis of Λ, the natural map Λ Zp H 1 (I K, µ p n) H 1 (I K, (Λ /(p n ))(1)) is an isomorphism, and we can pass this tensor product through an inverse limit to get H 1 (I K, Λ (1)) = Λ Zp H 1 (I K, Z p (1)).

9 The crystalline property of ρ is now going to be defined in terms of a description of H 1 (I K, Z p (1)) and the identification of ρ IK as a class in Λ Zp H 1 (I K, Z p (1)). (After making the definition, we will make it more concrete in terms of matrices.) We note at the outset that the definition we will give is in fact equivalent to a special case of a general notion of crystalline defined in p-adic Hodge theory, but we have avoided any discussions of p-adic Hodge theory and so will likewise have no need to delve further into the justification for our choice of terminology. A reader who pursues the subject in greater depth will eventually meet the general concept of crystalline, but it is logically unnecessary for our purposes. Let K denote the completion of the maximal unramified extension K un, so K = W (F) W (F) K where F is the finite residue field of K. Thus, I K = G K and O K is a complete discrete valuation ring with uniformizer given by one for O K. In particular, 1 + m K is p-adically separated and complete as a multiplicative Z p -module. By Kummer theory, (3.1) H 1 (G K, Z p (1)) = lim K /K un p n = Z p (1 + m K ) where the Z p -factor corresponds to powers of a fixed uniformizer of K (or K ). This direct product decomposition is not canonical: the direct factor of Z p depends on a choice of uniformizer. However, the Z p -hyperplane of 1-units 1 + m K (a multiplicative Z p -module) is canonical. Definition 3.1. The ordinary representation ρ : G K GL 2 (Λ ) is crystalline if its class in H 1 (I K, Λ (1)) lies in the Λ -hyperplane (1 + m K ) Zp Λ. Equivalently, ρ : G K GL 2 (E ) corresponds to a class in H 1 (I K, E (1)) lying in the E - hyperplane (1 + m K ) Zp E. In view of the formulation over E, the crystalline condition is intrinsic to the E -linear representation space for G K, so it does not depend on a specific choice of G K -stable Λ -lattice. Example 3.2. The concrete meaning of the crystalline condition is as follows. In terms of a choice of G K -stable Λ -lattice, consider ρ IK mod p n for each n 1. This is upper triangular unipotent, with upper-right entry given by a Λ -linear combination of 1-cocycles g g(u 1/pn )/u 1/pn on I K, with u O K. Loosely speaking, Kummer theory shows that ρ IK is given by a Tate-curve type of construction with Λ -coefficients, and the crystalline condition is that the q-parameter can be chosen to be a unit (or equivalent a 1-unit, since F is uniquely p-divisible). Now we fix a residual representation ρ 0 : G K GL 2 (k) with cyclotomic determinant, and we seek to study its ordinary crystalline deformations with coefficients in p-adic fields or valuation rings thereof. In order to make good sense of these notions in deformation theory, we need to generalize the definition of ordinary crystalline to the case of more general coefficients. Remark 3.3. We will later need to consider coefficients in rings like R[T ] that are not m R - adically separated and complete, so this will create some delicate problems when we work 9

10 10 with finite type R-schemes such as P 1 R that we want to use in the construction of a moduli scheme for an ordinary crystalline deformation problem. The creative use of proper (generally non-finite) R-schemes in Galois deformation theory is one of the innovations introduced by Kisin s work. Consider the universal deformation ρ : G K GL 2 (R) or universal framed deformation ρ : G K GL 2 (R ) of ρ 0 with cyclotomic determinant. We want to study the locus of ordinary crystalline points in MaxSpec(R E ). The formalism for this study will not really use the universality at all, so to keep the picture clear we now consider any continuous ρ : G K GL 2 (R) as at the outset such that det ρ is cyclotomic, and we continue to assume that the residue field k of R is finite. For any R-algebra A, let ρ A : G K GL 2 (A) denote the composition of ρ with R A on matrix entries. Note that there is no meaningful continuity condition for ρ A for general A, since we are not assuming that A carries an interesting topology compatible with the one on R. In the special case that A is an R/m n R -algebra, ρ A is continuous for the discrete topology on A (and the Krull topology on G K ) since ρ mod m n R is continuous for the discrete topology on R/m n R. Beware that if we take A = R/(pn ) with the discrete topology then ρ A is typically not continuous. It will therefore be important that we can work modulo powers of the maximal ideal of R and bootstrap back up to geometric objects over R via limit procedures. Example 3.4. Our work with ρ A for R/m n R-algebras A will involve some Galois cohomology with A-coefficients viewed discretely, so we record here the useful fact that for any Z/(p n )- module M viewed discretely (such as M = A) the natural map M Zp H 1 (I K, Z p (1)) H 1 (I K, M(1)) is an isomorphism. To prove this, we can use direct limits in M to reduce to the case when M is a finitely generated Z p -module. Hence, M is a finite direct sum of modules of the form Z/(p r ) with r n, so it suffices to treat the case M = Z/(p r ). Then the assertion is that the natural map H 1 (I K, Z p (1))/(p r ) H 1 (I K, µ p r) is an isomorphism for all r 1. For K denoting the discretely-valued completion of K un we have I K = G K and 1 + m K is p-adically separated and complete (as a multiplicative Z p -module), so Kummer theory and the description of H 1 (I K, Z p (1)) in (3.1) yields the result. In the special case that A is the valuation ring of a finite extension of E, we have defined what it means to say that ρ A is ordinary crystalline (in Definition 3.1). That definition involved the p-adic topology of the valuation ring. We wish to define this concept for R/m n R - algebras A, avoiding any use of nontrivial topologies on rings. Definition 3.5. Let A be an R-algebra killed by m n R for some n 1. The representation ρ A of G K on V A := A 2 is ordinary crystalline if there is a G K -stable A-submodule L A V A such that L A and V A /L A are locally free of rank 1 (equivalently, projective of rank 1) and:

11 (1) the I K -action on V A /L A is trivial, or equivalently the continuous action of G K on V A /L A is through an unramified character η : G K A and on L A is through χη 1 ; (2) under the valuation map K un Z, the class in H 1 (I K, A(1)) = H 1 (I K, Z p (1)) Zp A = (K un /(K un ) pn ) Z/(p n ) A describing ρ A IK is carried to 0 A (i.e., the class arises from integral units of K un ). We call the A-line L A V A an ordinary crystalline structure on ρ A. Remark 3.6. In the crystalline condition (2) in this definition, we have invoked the cohomology computation in Example 3.4. Also, in general there may be more than one choice of L A (if any exist at all!). For example, if A = k and ρ k has trivial G K -action (in particular, the mod-p cyclotomic character is trivial) then every line in V k = k 2 is an ordinary crystalline structure on ρ k. A fundamental insight of Kisin is that rather than trying to parameterize deformations which admit an additional structure (such as an ordinary crystalline structure) that may not be unique, it is better to parameterize the space of pairs consisting of a deformation equipped with such an additional structure. To make reasonable sense of a parameter space for such enhanced objects, we will have to leave the framework of complete local noetherian rings and instead work with certain proper schemes over such rings. The property in (2) in the preceding definition makes sense as a condition on classes in H 1 (I K, M(χ)) for any discrete Z/(p n )-module M for any n 1. (Note that M(χ) is a discrete I K -module, and even a discrete G K -module.) Definition 3.7. For any discrete Z/(p n )-module M equipped with a continuous unramified G K -action (not necessarily trivial), the subgroup H 1 crys(k, M(χ)) H 1 (K, M(χ)) consists of classes whose restriction to H 1 (I K, M(χ)) = M Zp H 1 (I K, Z p (1)) is killed by the valuation mapping H 1 (I K, Z p (1)) Z p defined by Kummer theory. (In other words, the I K -restriction is an M-linear combination of classes in H 1 (I K, Z p (1)) arising from integral units of the completion of K un.) It is immediate from the definition that if M = lim M i for a directed system of unramified discrete (Z/(p n ))[G K ]-modules M i then the equality lim H1 (K, M i (χ)) = H 1 (K, M(χ)) carries lim H 1 crys(k, M i (χ)) isomorphically onto H 1 crys(k, M(χ)). In other words, the formation of H 1 crys(k, M(χ)) is compatible with direct limits in M. This will be very useful for reducing some general assertions to the special case of M which are Z p -finite (whereas in applications we will need to work with M which are not Z p -finite, such as (R/m n R )[t]). Example 3.8. Isomorphism classes of pairs (ρ A, L A ) as in Definition 3.5 correspond to elements in H 1 crys(k, A(1)). 11

12 12 Example 3.9. It is important to link Definition 3.5 and Definition 3.1. For ρ : G K GL 2 (Λ ) as in Definition 3.1 we claim that it is ordinary crystalline in that initial sense (which can be checked over the fraction field E of Λ ) if and only if the artinian quotients ρ mod π n Λ of ρ are ordinary crystalline in the sense of Definition 3.5 with R = Λ (i.e., each ρ mod π n Λ admits an ordinary crystalline structure). It is obvious that if ρ is ordinary crystalline in the initial sense then each artinian quotient ρ mod π n Λ admits an ordinary crystalline structure. To go in reverse, suppose that every such artinian quotient admits an ordinary crystalline structure. Such structures are not unique in general, but since k is finite there are only finitely many such structures for each n 1. These finite non-empty sets form an inverse system in an evident manner, and so the inverse limit is non-empty. (This is an elementary fact since the inverse system is indexed by positive integers and not a general index set.) An element of the inverse limit is precisely the data of a saturated G K -stable Λ -line L in ρ such that (i) ρ mod L has trivial I K -action (as may be checked modulo π n for all n 1), and (ii) the class in H 1 (I K, Λ (1)) = Λ Zp H 1 (I K, Z p (1)) corresponding to (ρ IK, L) has image under the valuation mapping H 1 (I K, Λ (1)) Λ which vanishes (as this can also be checked modulo π n for all n 1). The conditions (i) and (ii) say exactly that ρ is ordinary crystalline in the sense of Definition 3.1. The proof of a later formal smoothness result over Λ will rest on: Lemma For any n 1, the functor M H 1 crys(k, M(χ)) on discrete unramified (Z/(p n ))[G K ]-modules is right-exact. This is analogous to the fact that H 1 (I K, M(χ)) = M Zp H 1 (I K, Z p (1)) (see Example 3.4) is right-exact in discrete p n -torsion abelian groups M with trivial I K -action. Proof. By discreteness we can express any M as a direct limit of Z p -finite G K -submodules, so any right exact sequence in M s is obtained as a direct limit of right-exact sequences of Z p -finite object. Thus, the compatibility with direct limits in M reduces the problem to right-exactness for M which are finite abelian p-groups. There are two ways to settle the finite case. In [3, 2.4.2], Kisin does some work with cocycles to derive an explicit description of H 1 crys(k, M(χ)) which makes the right-exactness evident by inspection. This is definitely the most elementary way to proceed. For the reader who doesn t like cocycle arguments and is familiar with the fppf topology, here is an alternative explanation in such terms. This explanation is longer, but may be seen as more conceptual (and clarifies the role of finiteness of the residue field). The finite discrete G K -module M(χ) has unramified Cartier dual, so it is the generic fiber of a unique finite flat O K -group scheme M(χ) with étale Cartier dual, and M(χ) is functorial in M. If 0 M 1 M 2 M 3 0 is an exact sequence of such unramified G K -modules then the complex of finite flat O K -group schemes 0 M 1 (χ) M 2 (χ) M 3 (χ) 0 is short exact (in particular, short exact as abelian sheaves for the fppf topology over O K ), as may be checked using the finite étale Cartier duals.

13 13 It can be shown by a Kummer-theoretic argument in the fppf topology that H 1 crys(k, M(χ)) = H 1 fppf(o K, M(χ) ), so it suffices to show that H 1 fppf (O K, M(χ) ) is right exact in M. Equivalently, a short exact sequence in M induces a right-exact sequence in H 1 fppf (O K, M(χ) ). The long-exactness of fppf cohomology then does the job provided that H 2 fppf (O K, M(χ) ) = 0 for any finite abelian p-group M equipped with an unramified continuous G K -action. The filtration by {p m M} reduces us to the case when M is p-torsion. If r = dim Fp M and k /k is a finite Galois extension which splits M then for the corresponding finite unramified extension K /K we have that M(χ) O K = µ r p. Thus, we have an O K -subgroup inclusion M(χ) Res OK /O K (M(χ) O K ) = Res O K /O K (µ r p), where Res OK /O K denotes Weil restriction of scalars. This latter operation represented pushforward at the level of fppf sheaves, and it is an exact functor because O K O K is finite étale (and hence a split covering étale-locally over Spec(O K )). We conclude that M(χ) is contained in the O K -group scheme T = Res OK /O K (G r m) which is an O K -torus (as we see by working étale-locally to split the covering Spec(O K ) Spec(O K )). Hence, we have a short exact sequence 1 M(χ) T T 1 where T := T /M(χ) is another O K -torus. Thus, using the resulting long exact sequence in fppf cohomology, to prove H 2 fppf (O K, M(χ) ) = 0 it suffices to prove that H 2 fppf (O K, T ) vanishes and that any O K -torus (such as T ) has vanishing degree-1 cohomology. For the latter, first recall that degree-1 cohomology with affine coefficients classifies fppf torsors, so the degree-1 vanishing amounts to the triviality of such torsors when the coefficients are smooth and affine with connected fibers (such as a torus). To build a section splitting such a torsor over O K it suffices (by smoothness of the coefficients, and the henselian property for O K ) to find a section over the residue field. That is, we are reduced to proving the vanishing of degree-1 cohomology over k with coefficients in a smooth connected affine group (such as a torus). This is Lang s theorem, since k is finite. Finally, to prove that H 2 fppf (O K, T ) is trivial, by using the definition of T this amounts to vanishing of H 2 fppf(o K, Res OK /O K (G m )). The exactness of the Weil restriction functor in this case implies (by a δ-functor argument) that H j fppf (O K, Res OK /O K ( )) H j fppf (O K, ) for all j 0. Hence, we just have to check the vanishing of H 2 fppf (O K, G m) By Grothedieck s work on Brauer groups, this is identified with Br(O K ), and since O K is henselian this in turn is identified with Br(k ). But k is finite, so its Brauer group vanishes. Now we can prove the main existence result for a proper (even projective) R-scheme that classifies ordinary-crystalline pushforwards of ρ.

14 14 Theorem For each n 1, the functor on R/m n R-algebras given by A {L A V A L A is an ordinary crystalline structure on ρ A } is represented by a closed subscheme X n P 1 R/m n R. There is a unique closed subscheme X P 1 R such that X mod mn R = X n for all n 1. Proof. By the universal property of the projective line, P n := P 1 R/m represents the functor n R carrying any R/m n R -algebra A to the set of locally free A-submodules L A V A = A 2 of rank 1 such that V A /L A is also locally free of rank 1. Over P n, consider the G K -action on OP 2 n defined by ρ mod m n R and the R/mn R -algebra structure on O P n. For each g G K and any A-point of P n, the condition that A-pullback of the g-action on OP 2 n preserves the corresponding A-line in A 2 is represented by a closed subscheme Z g of P n. To prove this, we may work Zariski-locally on P n so that the universal line subbundle is free and extends to a basis of O 2. Then the vanishing of the resulting lower left matrix entry function over the open in the base is what cuts out the g-stability condition over such an open locus in P n. These closed loci agree on overlaps and glue to the desired closed subscheme Z g of P n attached to g. Thus, the closed subscheme Z n = g Z g P n representing the condition of G K preserving the universal line subbundle of O 2. Consider the character η n : G K O(Z n ) describing the G K -action on the universal line subbundle over Z n. The Zariski-closed conditions η n (g) = 1 for all g I K cut out a closed subscheme Z n Z n which represents the additional condition that the universal line subbundle is not only G K -stable but has unramified G K -action. In other words, Z n represents the functor of ordinary structures on ρ n. Over Z n, consider the further condition that the ordinary structure is crystalline. That is, for an A-point of Z n, we consider the property that the resulting A-line L A in ρ A is an ordinary crystalline structure. The map H 1 (I K, A(1)) A defined by the valuation K un Z carries the class of (ρ A, L A ) to an element a A, and this construction is functorial in A. Hence, by (the proof of) Yoneda s Lemma it defines a global function h n on Z n. The zero scheme of h n on Z n is clearly the desired X n. Having constructed X n P n for each n 1, the behavior of moduli schemes with respect to base change implies that the isomorphism P n P n+1 R/m n+1 R (R/m n R) carries X n over to X n+1 mod m n R. In other words, {X n} is a system of compatible closed subschemes of the system {P n } of infinitesimal fibers of the proper morphism P 1 R Spec R over the complete local noetherian ring R. Now comes the deepest step: by Grothendieck s formal GAGA (EGA III 1, 5), if R is any complete local noetherian ring and P is any proper R-scheme, then the functor Z {Z mod m n R} from closed subschemes of P to systems of compatible closed subschemes of the infinitesimal fibers of P over Spec R is a bijection. (Even for P = P 1 R this is not obvious, and it fails

15 miserably if we consider the affine line instead of the projective line.) existence and uniqueness of the desired X. 15 Thus, we get the Remark Although each infinitesimal fiber X n of X over Spec R has moduli-theoretic meaning for points valued in arbitrary R/m n R-algebras, we do not claim that X has a convenient moduli-theoretic meaning for its points valued in arbitrary R-algebras. In particular, X E = X R R E has no easy interpretation. Nonetheless, it is X E which will be of most interest to us. Thus, to work with X E we need a way to understand its properties by studying the X n s. This problem will be taken up in the next section. 4. Properties and applications of the ordinary crystalline moduli scheme The construction of the proper R-scheme f : X Spec R is indirect, as formal GAGA is very abstract, but we can artfully use the construction to infer global properties of X which will be especially useful for the study of X E. Our analysis rests on the following hypothesis which is in force throughout this section (unless we say otherwise): Assume that det ρ : G K R is cyclotomic and that ρ is the universal framed cyclotomicdeterminant deformation ring of its reduction ρ k. If ρ k has only scalar endomorphisms, we also allow that (ρ, R) is the universal deformation of ρ k with cyclotomic determinant. Proposition 4.1. The Λ-scheme X is regular and flat, and X mod π is reduced. Proof. By Proposition 2.1, Lemma 2.2, and Proposition 2.3, it suffices to prove that X(B) X(B/J) is surjective for every artin local finite Λ-algebra B. Choose such a B, and let k be its residue field, so B is canonically an algebra over Λ = W (k ) W (k) Λ. It is harmless to make the finite étale scalar extension by Λ Λ throughout (this is compatible with the formation of X) to reduce to the case k = k. The maximal ideal of B is nilpotent, say with vanishing nth power for some n 1, so the map of interest on points of X coincides with the analogue for X n. Thus, the task is to show that if L is an ordinary crystalline structure on ρ B/J then it lifts to one on ρ B. Let x 0 X be the closed k-point corresponding to the specialization (ρ k, L k ) of (ρ B/J, L) over the residue field k of B. Our problem is equivalent to showing that any local Λ-algebra map OX,x 0 B/J lifts to B. Thus, it is sufficient (and even necessary) to prove that OX,x 0 is a formal power series ring over Λ. To do this, we need to give a deformation-theoretic interpretation of this completion. Since x 0 is a closed point, R OX,x 0 is a local map and its reduction modulo m n R recovers OX n,x 0 due to the relationship between X and the X n s. But X n is an actual moduli scheme over the ring R/m n R (unlike X over R). In view of the assumed universal property of (ρ, R), it follows that OX n,x 0 is the deformation ring for ordinary crystalline structures lifting L k on cyclotomic-determinant deformations of ρ k (possibly with framing) having coefficients in Λ-finite artin local rings whose nth power vanishes. Hence, OX,x 0 is the analogous formal deformation ring for arbitrary Λ-finite artin local coefficients (without restriction on the nilpotence order of the maximal ideal).

16 16 Our problem is therefore to prove that there is no obstruction to infinitesimal deformation of (ρ k, L k ) as ordinary crystalline structures with cyclotomic determinant. (There is no obstruction when we impose the additional data of a framing, as that amounts to simply lifting some bases through a surjection of finite free modules.) That is, given such a representation over B/J we wish to lift it to one over B. The given representation with ordinary crystalline structure over B/J has diagonal characters {χη 1, η} for some unramified continuous η : G K (B/J). Since G K /I K = G k = Ẑ, we can lift η to an unramified continuous η : G K B (choose an arbitrary lift of η(frob k ) (B/J) ). We claim that this lifts to an ordinary crystalline deformation to B with diagonal characters {χ η 1, η}. Thinking in terms of upper-right matrix entries, we have to prove the surjectivity of the natural map H 1 crys(k, B(χ η 2 )) H 1 crys(k, (B/J)(χη 2 )). For M = B( η 2 ) we have M/JM = B(η 2 ), and these are unramified discrete G K -modules killed by a power of p. By Lemma 3.10, the natural map is surjective, so we are done. H 1 crys(k, M(χ)) H 1 crys(k, (M/JM)(χ)) Proposition 4.2. The natural map f E : X E Spec(R E ) is a closed immersion, and this regular closed subscheme meets MaxSpec(R E ) in precisely the set of closed points x MaxSpec(R E ) such that ρ x : G K GL 2 (E(x)) is ordinary crystalline in the sense of Definition 3.1. In particular, the locus of ordinary crystalline points in MaxSpec(R E ) is Zariskiclosed. Proof. By Proposition 2.5, it suffices to prove that f E is injective on C-valued points for all E-finite C. We may assume C is local, so its residue field E is E-finite. By Hensel s Lemma and the separability of E /E, the E-algebra structure on C uniquely extends to an E -algebra structure lifting the residue field. Thus, C = E I for the nilradical I of C. In particular, if Λ denotes the valuation ring of E then Λ I is a local Λ -subalgebra of C, though it is generally not Λ -finite since the E -vector space I is generally nonzero. To prove that the map X(C) (Spec R)(C) between sets of C-valued points over Λ is injective, we can first replace f : X Spec R with its scalar extension by Λ Λ. This scalar extension is compatible with the formation of X (as may be checked on infinitesimal fibers over Spec R), so we may assume E = E. We fix a map φ : Spec C Spec R over Λ and seek to prove that it has at most one lift to a map φ : Spec C X. Consider the Λ-algebra map Λ[x 1,..., x m ]/(f 1,..., f s ) = R C = E I corresponding to φ. Passing to the quotient C/I = E, this map carries each x j to (c j, y j ) for some y j I and c j (π). Thus, we can make the formal change of parameters x j x j c j to get to the case when x j y j I for all j. Since I N = 0 for some large N, any monomial in the x s with large enough degree maps to 0 in C. Hence, the image of R in C is contained in C := Λ J for a finite Λ-submodule J I, and we can increase J so that C is a local finite flat Λ-subalgebra of C. Note that C E = C.

17 Now consider any φ lifting φ. The restriction of φ to the closed point of Spec C is a map φ 0 : Spec E X over the specialization φ 0 : Spec E R over Λ. This latter specialization is a Λ-algebra map R E and hence lands in Λ. This resulting Λ-valued point of Spec R uniquely lifts to a Λ-valued point of the R-proper X extending φ 0, due to the valuative criterion for properness. Consider an open affine Spec A in X around the image of the Λ- valued point of X extending φ 0. This open affine contains the image of φ since E = C/I with I nilpotent. The resulting Λ-algebra map A C = E I with A a finite type R-algebra lands in Λ I by the choice of A, and so lands in C upon taking J big enough. We have now constructed an R-map Spec C X that serves as an integral model (over Λ) for φ. The choice of J can always be increased even further, so to prove the uniqueness of φ (if one exists) it suffices to consider a pair of R-maps Spec C X over a common local Λ-map Spec C R, and to show that the resulting generic fiber maps Spec C X coincide. By locality of the Λ-map R C to the Λ-finite C, a cofinal system of open ideals in C is given by the m n C. R Using the formal GAGA construction of X from the X n s, it follows that the pair of maps Spec C X corresponds to a pair of ordinary crystalline structures on ρ ec (i.e., compatible such structures over each artinian quotient of C). These coincide provided that the resulting pair of filtrations on ρ C coincide, since a C-line in C 2 is uniquely determined by the associated C-line in C 2 (via saturation of Λ-finite submodules in finitedimensional E-vector spaces, as C is finite flat over Λ and C E = C). Thus, the injectivity of f E on C-points is reduced to proving that if G K GL 2 (C) is a homomorphism admitting an upper triangular form ( ) χη 1 0 η relative to some C-basis with η : G K C unramified then the χη 1 -line is uniquely determined. This C-line is precisely the locus of vectors on which I K acts through χ. Indeed, to prove this it suffices to check that in the quotient C-line η the space of χ-isotypic vectors for the I K -action vanishes. Since χ is valued in E and C has an E-linear filtration by ideals with successive codimension 1 over E, we just need to observe that χ 1 in E since char(e) = 0. (In more sophisticated p-adic Hodge theory settings, the analogue of this step requires results such as Tate s isogeny theorem for p-divisible groups over Λ: uniqueness results for integral structures in case of generic characteristic 0.) To identify the closed points of X E within MaxSpec(R E ), we will use Example 3.9. Closed points of X E are obtained from E -valued points for finite extension fields E /E; let Λ be the valuation ring of E. The preceding argument with the valuative criterion for properness shows that any E -valued point of X E uniquely extends to a Λ -valued point of X over Spec R, and such a point corresponds precisely to a filtration on ρ E as in Definition 3.1, due to Example 3.9. This proves that the closed points of X E are the ordinary crystalline points of MaxSpec(R E ). 17